By Stephen M. Silverman
Updated March 10, 2003 01:15 PM

Joaquin Phoenix had better bone up on his baritone.

The Oscar-nominated “Gladiator” actor, 28, is set to portray country legend Johnny Cash, while “Legally Blonde” star Reese Witherspoon, 26, will play Cash’s wife and bandmate June Carter in the upcoming feature movie “Walk the Line,” says the Hollywood Reporter.

The musical biography is expected to begin shooting this fall, led by “Girl, Interrupted” and “Kate & Leopold” director James Mangold, 38, whose thriller “Identity,” with John Cussack and Ray Liotta, is scheduled for release next month.

“Walk the Line” is described by the trade paper as a nontraditional biopic that will deal with Cash’s younger years in his native Arkansas, his move to Memphis in the ’50s and the rise in mainstream popularity of country music (which also happened to coincide with the rise of rock ‘n’ roll).

Cash, now 70, had his first hit in 1955 for Sun Records, “Cry Cry Cry.” His “I Walk the Line” was another hit, in 1956.

The movie is also said to examine the man in black’s drug problems, which he reputedly overcame through the help of his wife, who is now 73.

The Reporter says the filmmakers have obtained cooperation and access to the couple’s life story and the rights to use Cash’s original music in the film.

At the moment, Cash, who suffers with diabetes and asthma, and whose health of late has been precarious, has scored another hit single with “Hurt,” his haunting cover of the Nine Inch Nails song.

Before Witherspoon steps into June Cash’s shoes, she has committed to the sequel “Legally Blonde 2: Red, White & Blonde.”